1. Field of the Invention
This invention relates to a toner for developing electrostatic images, suited for heat fixing, used in electrophotography, electrostatic recording and magnetic recording. It also relates to a heat fixing method for fixing the toner by heating.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are hitherto known for electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by fixing means such as heat, pressure or solvent vapor. The toner that has not transferred to and has remained on the photosensitive member is cleaned by various means, and then the above process is repeated.
In recent years, such copying apparatus have begun to be used not only as office copying machines for merely taking copies from originals as is done commonly, but also as printers which are output means of computers, or in the field of personal copy making device.
Under such circumstances, the apparatus are severely sought to be made more small-sized and light-weight, and also to be made more high-speed and more highly reliable. Thus, machines are now constructed with simpler constituents in various points. As a result, toners are required to have a higher performance, and it has become impossible to accomplish superior machines unless improvements in performance of toners can be achieved.
Various methods or devices have been developed in relation to the step of fixing a toner image to a sheet such as paper. For example, the pressure heat system using a heat roller and the heat fixing system where a transfer medium is brought into close contact with a pressure member interposing a film between them are available.
Such heating systems using a heat roller or a film are methods of carrying out fixing by causing the toner image surface of an image-receiving sheet to pass the surface of a heat roller whose surface is formed of a material having releasability to toner (toner-releasing characteristics) while the former is brought into contact with the latter. Since in this method the surface of the heat roller comes into contact with the toner image of the image-receiving sheet under application of a pressure, a very good thermal efficiency can be achieved when the toner image is melt-adhered onto the image-receiving sheet, so that fixing can be carried out rapidly. Thus, this method is very effective in electrophotographic copying machines. In these methods, however, since the surface of the heat roller or film comes into contact with the toner image in a molten state, part of the toner image may adhere and transfer to the surface of the fixing roller or film, which is re-transferred to the subsequent image-receiving sheet to cause what is called the offset phenomenon, resulting in a contamination of the image-receiving sheet. Thus, it is considered to be one of essential conditions in the heat fixing system that no toner adheres to the surface of the heat fixing roller.
For the purpose of not causing the toner to adhere to the surface of a fixing roller, measures have been hitherto taken that the roller surface is formed of a material such as silicone rubber or fluorine resin, having an excellent releasability to toner, and, in order to prevent offset and to prevent fatigue of the roller surface, its surface is further covered with a thin film formed using a fluid having a good releasability as exemplified by silicone oil. Although this method is very effective from a viewpoint of inhibiting the offset of toner, it requires a device for feeding an anti-offset fluid, and hence has the problem that the fixing device becomes complicated.
This is in the opposite direction to the demand for small size and light weight. In some instances, the silicone oil is evaporated by heat to contaminate the interior of the machine. Now, based on the idea that the fluid for preventing offset should be fed from the the inside of a toner without the use of any apparatus for feeding silicone oil, a method has been proposed in which a release agent such as a low-molecular weight polyethylene or a low-molecular weight polypropylene is added in the toner. Addition of such a release agent in a large quantity in order to attain a sufficient effect may cause filming to the photosensitive member or cause a contamination of the surface of a toner carrying member such as a carrier or a sleeve, so that toner images may be deteriorated to raise a problem in practical use. Thus the release agent is added to the toner in such a small amount that may not cause the deterioration of toner images, where a releasing oil is fed in a little amount and a device for cleaning the toner that may cause offset by using a member such as a web of a wind-up type is used together.
However, taking account of the recent demand for small size, light weight and high reliability, it is necessary and preferred to remove even such a supplementary device. Accordingly, no countermeasure can be completely taken unless the fixing performance and anti-offset of the toner are further improved. It is difficult to achieve the improvement unless binder resins and release agents for toners are further improved.
Incorporating a wax into toners as a release agent is disclosed, for example, in Japanese Patent Publications No. 52-3304, No. 52-3305 and No. 57-52574.
Techniques for incorporating waxes are also disclosed in Japanese Patent Applications Laid-open No. 3-50559, No. 2-79860, No. 1-109359, No. 62-14166, No. 61-273554, No. 61-94062, No. 61-138259, No. 60-252361, No. 60-252360 and No. 60-217366.
Waxes are used to improve anti-offset properties of toners in low-temperature fixing or high-temperature fixing or to improve fixing performance in low-temperature fixing. However, while these performances are improved, blocking resistance may become poor, developing performance may become poor when toner is exposed to heat caused by an in-machine temperature rise, or wax blooming may occur when toners are left for a long period of time, to make developing performance poor.
None of conventional toners have satisfied all requirements of these aspects, and have caused some problem. For example, some have good high-temperature anti-offset properties but have not so definitely good low-temperature anti-offset properties, some have good low-temperature anti-offset properties and low-temperature fixing performance but have a little poor blocking resistance to undesirably cause a lowering of developing performance due to in-machine temperature rise, or some can not achieve anti-offset properties in high-temperature fixing and low-temperature fixing at the same time.
Toners containing a low-molecular weight polypropylene (e.g., VISCOL 550P, 660P, etc., produced by Sanyo Chemical Industries Co., Ltd.) are commercially available, but it is sought to provide toners more improved in low-temperature anti-offset properties and also improved in fixing performance.
In order to improve fixing performance of toners, binder resins contained in toners are also improved. When only the fixing performance is taken into account, the binder resins are required to have lower molecular weight and glass transition point. This, however, causes a lowering of high-temperature anti-offset properties and blocking resistance.
To overcome such disadvantages, Japanese Patent Publication No. 51-23354, for example, proposes a toner in which a cross-linked polymer (a vinyl-type polymer) is used as a binder resin, Japanese Patent publication No. 55-6895 proposes a toner containing a binder resin having xcex1,xcex2-unsaturated ethylene monomers as component units and made to have a broader molecular weight distribution of 3.5 to 40 in the ratio of weight average molecular weight to number average molecular weight, and Japanese Patent Application Laid-open No. 56-16144 proposes a toner containing a binder resin having in a viny polymer a peak in each of its low-molecular weight region and high-molecular weight region on a GPC chromatogram. It is true that these toners proposed have achieved both high-temperature anti-offset properties and fixing performance at the same time to a certain extent, but further improvements are sought.
In place of these vinyl type, addition polymerization type binder resins, condensation polymerization type polyesters have been proposed in the past.
For example, Japanese Patent Application Laid-open No. 59-7960 discloses a toner comprising an improved specific polyester as a binder resin. This has certainly achieved a low-temperature fixing performance superior to vinyl type resins, but its release component wax has a poor dispersibility to raise a problem of poor anti-offset properties.
Japanese Patent Application Laid-open No. 5-197192 also discloses a toner containing a hydrocarbon wax having specific thermal properties. This toner containing a hydrocarbon wax having specific thermal properties can impart preferable thermal properties to toners and hence have superior fixing performance, anti-offset properties and blocking resistance in low-temperature fixing. However, in order to make more effective the hydrocarbon wax having specific thermal properties, it is desired for the hydrocarbon wax to have a much better dispersibility in the binder resin, and there is room for further improvement.
Japanese Patent Application Laid-open No. 5-249735 discloses a toner containing a (styrene type) binder resin having a functional group, and a hydrocarbon wax, where the elasticity modulus and thermal properties of the toner are defined so that its fixing performance, anti-offset properties and blocking resistance can be improved.
There, however, is a limit in the improvement of performances (in particular, fixing performance) if it relies on only these known techniques. In order to well bring out the performance of polyesters which is matched to low-temperature fixing, it is sought to make more improvements while maintaining other toner performances such as blocking resistance and developing performance.
An object of the present invention is to provide a toner for developing an electrostatic image, having solved the problems as discussed above, and a method for fixing such a toner by heating.
Another object of the present invention is to provide a toner having superior fixing performance and anti-offset properties in low-temperature fixing, and a method for fixing such a toner by heating.
Still another object of the present invention is to provide a toner having superior anti-offset properties in high-temperature fixing, and a method for fixing such a toner by heating.
A further object of the present invention is to provide a toner having a superior blocking resistance, causing no deterioration of developing performance even when left for a long period of time, and a method for fixing such a toner by heating.
A still further object of the present invention is to provide a toner having a superior durability against in-machine temperature rise, and a method for fixing such a toner by heating.
A still further object of the present invention is to provide a toner for developing an electrostatic image, having a superior rise of charging in high-temperature fixing, and a method for fixing such a toner by heating.
To achieve the above objects, the present invention provides a toner for developing electrostatic images, comprising a binder resin and a wax, wherein;
the binder resin contains as a primary component a polyester resin having a soft segment; and
the wax has, in its endothermic peaks at the time of temperature rise and exothermic peaks at the time of temperature drop in the DSC curve measured using a differential scanning calorimeter;
(i) an endothermic onset temperature within the range of from 50xc2x0 C. to 110xc2x0 C.;
(ii) at least one endothermic peak P1 within the range of from 70xc2x0 C. to 130xc2x0 C. at the time of temperature rise; and
(iii) a maximum exothermic peak at the time of temperature drop, within the range of plus-minus 9xc2x0 C. of the endothermic peak P1.
The present invention also provides a heat fixing method comprising fixing a toner image on a recording medium by a heat fixing means, wherein;
the toner image is formed by a toner having at least a binder resin and a wax;
the binder resin contains as a primary component a polyester resin having a soft segment; and
the wax has, in its endothermic peaks at the time of temperature rise and exothermic peaks at the time of temperature drop in the DSC curve measured using a differential scanning calorimeter;
(i) an endothermic onset temperature within the range of from 50xc2x0 C. to 110xc2x0 C.;
(ii) at least one endothermic peak P1 within the range of from 70xc2x0 C. to 130xc2x0 C. at the time of temperature rise; and
(iii) a maximum exothermic peak at the time of temperature drop, within the range of plus-minus 9xc2x0 C. of the endothermic peak P1.